Protective circuit for the input-side protection of an electronic device operating in the maximum frequency range

ABSTRACT

The invention relates to a protective circuit ( 10 ) for the input-side protection of an electronic device ( 30 ) operating in the maximum frequency range from high-power interfering impulses in the working frequency range of the device ( 30 ), said protective circuit ( 10 ) between an input ( 11 ) and an output ( 18 ) comprising a first limiting circuit ( 12 ) having at least one gas discharge tube (GDT 1,  GDT 2 ) for limiting high interference powers and a second limiting circuit ( 14, 16 ) disposed behind the first limiting circuit ( 12 ), said second limiting circuit having semi-conductor components (D 1, . . . ,  D 4 ) for limiting smaller interference powers. Protection from particularly high interference powers is achieved in that the first limiting circuit ( 12 ) comprises two gas discharge tubes (GDT 1,  GDT 2 ) connected in parallel and preferably identical.

In wireless communication, in the case of radar installations or otherreceiving devices which operate at microwave frequencies, and whoseinput is normally coupled to an antenna, high-power interference pulsesor frequency bursts, with steep rising flanks can deliberately orinadvertently be applied to the input of the appliance, whosefrequencies fall in the operating frequency range of the appliance,where they temporarily or permanently affect the sensitive input stagesor, furthermore, cause damage.

In the past, it has therefore variously been proposed that applianceinputs such as these be protected by means of protective circuitsupstream of the input, which reliably limit or attenuate such fasthigh-power interference pulses without noticeably impeding the wantedsignals being fed into the appliance input. In this case, a plurality oflimiter circuits, which cover a coarse and a fine range of theinterference power, are frequently connected in series, for example withgas discharge paths being used for the coarse range and limitingsemiconductor components being used for the fine range.

By way of example, CH-A5-654 149 discloses a protective circuit againstNEMPs, in which a gas cell as coarse protection means and a varistor asfine protection means are connected in series via a frequency-selectivedelay element.

DE-A1-34 25 296 discloses a protective apparatus for electronicappliances, in which a coarse protective device with a gas dischargepath and a fine protective device which operates with semiconductorelements are accommodated in separate housings and are connected to oneanother via a relatively long connecting cable. DE A1-36 26 800 alsodiscloses a comparable solution.

DE A1-39 07 199 discloses an overvoltage protective device for coaxialcables, in which a three-electrode suppressor is used for coarseprotection, and a combination of switching diodes and limiter diodes isused for fine protection. The three-electrode suppressor is insertedinto a printed circuit board on which the other circuit elements arealso arranged. The end-face electrodes of the three-electrode suppressorare connected to the inner conductor and outer conductor of the coaxialcable, while the center electrode is connected to a separate groundline.

EP-A1-0 855 758 discloses a circuit arrangement for protection of RFinput circuits of telecommunication appliances, in which a fineprotective circuit having a varistor and a limiter diode is providedbehind a gas capsule suppressor or a lambda/4 short-circuit line.

Finally, it has been known for a long time for PIN diodes to be used forlimiting interference pulses at the input of appliances which operate inthe microwave range, and these are distinguished by very small internalcapacitances and short response times (see for example U.S. Pat. No.5,345,199 or WO-A1-2006/124104).

Other protective methods relate to quick active regulation of a limitingelement in the input of the appliance (US-A1-2006/0293020).

The known protective circuits and protective methods are not sufficientfor reliably limiting or attenuating very high power interferencepulses, for example those transmitted by pulsed radars or specificinterference pulse generators.

The object of the invention is to specify a protective circuit whichavoids the disadvantages of known protective circuits and isdistinguished by a simple, compact and robust design, as well asreliable limiting of fast and, in particular, high-power interferencepulses in the microwave band.

The object is achieved by the totality of the features of claim 1. Oneessential feature for the invention is a limiter circuit, arranged atthe input of the protective circuit, for the coarse power range, whichcomprises two parallel-connected gas discharge tubes which are connectedin parallel in a specific manner. The two gas discharge tubes, which arepreferably of the same type and are arranged one behind the otherconnected in parallel allow the isolation with respect to interferencepowers at the input to be improved by an order of magnitude (a factor ofapproximately 10) by very simple means and with a very compact design.

For optimum limiting, it is advantageous for the two gas discharge tubesto be coupled to one another as closely as possible, with regard totheir ignition. According to one refinement of the invention, this isachieved in that the two gas discharge tubes have a common gas space.

It is particularly advantageous for the two gas discharge tubes to bephysically combined in one gas capsule, which comprises a cylindricalgas space with two mutually opposite end-face electrodes and a centerelectrode which is arranged in the center between the two end-faceelectrodes, wherein the end-face electrodes each form one electrode ofthe two gas discharge tubes, and the center electrode forms the otherelectrode of the two gas discharge tubes. This makes it possible for onegas discharge tube to likewise be ignited comparatively quickly when the(adjacent) other gas discharge tube ignites, because of the plasma thatis created in the gas space.

A bridging circuit which is in the form of a T-element with twoinductances and one capacitance is preferably arranged between theend-face electrodes of the gas capsule, in order to pass on the wantedsignals.

The coupling between two gas discharge tubes is improved further if theend-face electrodes extend so far in the axial direction into the gasspace that their ends are covered by the center electrode, when seen inthe radial direction. This results in the discharge paths of the twotube or electrode systems being directly adjacent, and influencing oneanother particularly intensively.

In order to ensure that the proximity of the two gas discharge tubesdoes not lead to undesirable electromagnetic coupling between the inputand the output of the limiter circuit, means are preferably provided forelectromagnetically decoupling the two gas discharge tubes.

In particular, for electromagnetic decoupling of the two gas dischargetubes, the gas capsule is installed with its cylinder axis essentiallyat right angles to the plane into an electrically conductive flat plate,such that the plate is located between the end-face electrodes of thegas capsule. The plate then has a shielding effect.

Another refinement of the invention is characterized in that PIN diodesare used as semiconductor components for limiting relatively lowinterference power levels, with the PIN diodes preferably beingconnected back-to-back in parallel, in pairs.

A further refinement of the invention is distinguished in that a thirdlimiter circuit is arranged behind the second limiter circuit andlikewise comprises semiconductor components, in that PIN diodes are ineach case connected back-to-back in parallel, in pairs as semiconductorcomponents in the second and the third limiter circuit, and in that thePIN diodes in the third limiter circuit have a lower power and respondmore quickly than the PIN diodes in the second limiter circuit. Thethird limiter circuit reduces the response delay of the protectivecircuit even further. At the same time, the limiting range is extendedtoward lower power levels.

In order to achieve a compact design and good heat dissipation from thelimiter elements, it is advantageous if the PIN diodes are arranged asunpackaged semiconductor chips directly on a thin ceramic substratewhich is mounted flat on a cold plate for heat dissipation.

Another refinement of the invention is distinguished in that in order toensure the response of the limiter circuits, a decoupling circuit is ineach case arranged between the limiter circuits and between the lastlimiter circuit and the output, wherein, in particular, the decouplingcircuit which is arranged between the first and the second limitercircuit comprises a series circuit formed from a capacitance and atransmission line, and the decoupling circuits in front of and behindthe third limiter circuit each comprise a transmission line.

The invention will be explained in more detail in the following textwith reference to exemplary embodiments and in conjunction with thedrawing, in which:

FIG. 1 uses two figure elements to show a block diagram of a protectivecircuit according to one exemplary embodiment of the invention (FIG. 1a), and the arrangement of the protective circuit from Figure la infront of an appliance to be protected (FIG. 1 b);

FIG. 2 shows the internal design of the first limiter circuit shown inFigure la, with a gas capsule with a center electrode as a limitingelement;

FIG. 3 shows the circuit of the two other limiter circuits from Figurela, in each case with a pair of PIN diodes, connected back-to-back inparallel, as limiting elements;

FIG. 4 uses two figure elements to show the design of the decouplingcircuit between the first two limiter circuits (FIG. 4 a) and thedecoupling circuits in front of and behind the third limiter circuit(FIG. 4 b);

FIG. 5 shows the internal design of a preferred gas capsule with endelectrodes which extend into the area of the center electrode;

FIG. 6 shows a preferred circuit design for the protective circuit shownin FIGS. 1 to 4;

FIG. 7 shows an example of a diagram of power limiting by a gasdischarge tube;

FIG. 8 shows an example of a diagram of power limiting by two gasdischarge tubes arranged one behind the other in parallel; and

FIG. 9 shows an example of a diagram of power limiting by the protectivecircuit shown in FIG. 1 a.

FIG. 1 uses two figure elements to show a block diagram of a protectivecircuit according to one exemplary embodiment of the invention (FIG. 1a) and the arrangement of the protective circuit from FIG. 1 a in frontof an appliance to be protected (FIG. 1 b). The protective circuit 10 inthe exemplary embodiment has an input 11 and an output 18, which may bein the form of coaxial plug connections. By way of example, the input 11may be a Type N connector, and the output 18 a Type SMA. Plug connectorssuch as these for frequencies in the GHz range are available, forexample, from the applicant.

Three limiter circuits 12, 14 and 16 are arranged in series between theinput 11 and the output 18, and are connected to one another and to theoutput 18 by decoupling circuits 13, 15 and 17. The decoupling circuits13, 15 and 17 ensure that, when the fastest reacting third limitercircuit is activated, sufficient voltage is dropped to then subsequentlyactivate the second limiter circuit 14 and the first limiter circuit 12.

The power ranges and response times of the limiter circuits 12, 14 and16 are graduated, with the first limiter circuit 12 having the highestpower range and the slowest response time, while the third limitercircuit 16 is distinguished by the lowest power range and the fastestresponse time. The third limiter circuit 16 therefore extends the scopeof protection of the appliance (30 in FIG. 1 b) toward lower powerlevels and faster rising flanks, and therefore improves the effect ofthe protective circuit, irrespective of the actual configuration of thefirst limiter circuit 12. As shown in FIG. 1 b, the protective circuit10 is connected in front of the input 31 of the electronic appliance 30to be protected, and therefore protects the appliance againstoverloading by fast and high-power interference pulses.

If just one simple typical gas discharge tube of the conventional typewere to be used in the first limiter circuit 12, this would result in arelationship between the input power present at the input 11 and theoutput power that appears at the output 18, as is illustrated in thediagram in FIG. 7. The output power rises linearly with the input powerwith the gradient 1, until the gas discharge tube ignites at about 60Watts, and limits the output power to less than 10 Watts. As the inputpower rises further, the output power then also continues to rise, as aresult of which, at input power levels of several 1000 Watts, such asthose which can occur in extreme cases, the protection of the appliance30 is no longer ensured.

It has now surprisingly been found that a simple parallel circuit of twogas discharge tubes (GDT1 and GDT2 in FIG. 1 a) in the first limitercircuit 12 provides protection against 10-times higher input powerlevels, provided that the ignition responses of the two gas dischargetubes are sufficiently closely coupled to one another. This then resultsin a relationship between the input power and the output power as isshown in the diagram in FIG. 8, that is to say even input power levelsof 3000 Watts and more are reliably limited to output power levels ofabout 30 Watts, before the output power rises again with the inputpower.

In order to achieve close coupling in a simple manner between the twogas discharge tubes (GDT) arranged one behind the other in parallel, aceramic three-electrode gas discharge tube (gas capsule 19) is used asshown in FIG. 2 and FIG. 5, and is connected such that the two end-faceelectrodes 20, 22 respectively form one electrode of the two gasdischarge tubes GDT1, GDT2, and the center electrode 21 forms the otherelectrode of the two gas discharge tubes GDT1, GDT2. The type of gascapsule 19 illustrated in FIG. 5 is preferably used in this case, inwhich the end-face electrodes 20 and 22 project so far into the interiorof the gas space 23 in the axial direction that they are covered by the(annular) center electrode 21 in the radial direction at the ends. Thisresults in very close coupling between the discharge paths of theelectrode pairs 20, 21 and 22, 23, and this leads to the ignition of therear gas discharge tube GDT2 being triggered by the already ignitedfirst gas discharge tube GDT1, and in this being done with only a shorttime delay.

As can be seen in FIG. 2, the three-electrode gas capsule 19 isinstalled in the protective circuit 10 such that the path which isformed between the two end-face electrodes 20, 22 interrupts the innerconductor of the protective circuit 10, while the center electrode 21 isgrounded. In order to bridge the interruption, a bridging circuit isarranged between the end-face electrodes 20 and 22, in the form of aT-element with two inductances L2 and L3 and a capacitance C2.Furthermore, the capacitances C1 and C3 of the discharge paths of thegas capsule 19 as well as the inductances L1 and L4 of the supply linesto the end-face electrodes 20 and 22 play a part in terms ofradio-frequency technology.

The second limiter circuit 14 and the third limiter circuit 16 aredesigned as shown in FIG. 3: two PIN diodes D1, D2 (and D3, D4 in FIG.6) of the same type are connected back-to-back in parallel in them. Byway of example, glass-passivated chips from the Tyco Company or M/A-Comcan be used as PIN diodes. In this case, the inductances L5, L6 and L7of the supply lines are also important for radio-frequency purposes, andthese are governed by the bonding wires 27, which can be seen in FIG. 6,of the PIN diode chips. The decoupling circuits 13, 15 and 17 which areprovided between the limiter circuits 12, 14 and 16 and after thelimiter circuit 16 are internally designed as shown in FIGS. 4 a and b:the limiter circuit 13 comprises a series circuit of a capacitance C4and a transmission line T1, and the limiter circuits 15 and 17 eachcontain a transmission line T2.

Since the protective circuit 10 must be designed for the GHz range andmust process considerable power levels in the event of interference, thedesign of the limiter circuits 12, 14 and 16 is of considerableimportance. FIG. 6 shows one exemplary embodiment of an appropriatecircuit design 24, in a simplified form. The circuit design comprises apreferably metallic cold plate 26 with good thermal conductivity and ahigh heat capacity, which can absorb and dissipate the heat that iscreated. A printed circuit board 25 is arranged on the upper face of thecold plate 26, and is used for wiring of the circuit. The printedcircuit board 25 has a cutout 32 which leaves free part of the area ofthe surface of the cold plate 26. Thin, electrically insulating ceramicsubstrates 28, 29 are arranged in the cutout 32, closely thermallycoupled to the cold plate 26, and the PIN diode pairs D1, D2 and D3, D4of the limiter circuits 14 and 16 are accommodated on their surface, inthe form of unpackaged semiconductor chips. The size relationships ofthe chips shown in

FIG. 6 have in this case been illustrated in an exaggerated form, inorder to improve clarity. The PIN diodes D1, . . . , D4 are adhesivelybonded or soldered on the lower face to the ceramic substrates 28, 29.The bonding pads on the upper faces are connected by means of bondingwires 27 to corresponding conductor tracks on the printed circuit board25.

A particular type of installation is envisaged for the gas capsule 19:corresponding openings are left free one above the other in the coldplate 26 and in the printed circuit board 25, through which thecylindrical gas capsule 19 can be passed, with its cylinder axis atright angles to the plane of the plate. In this case, the centerelectrode is located at approximately the same height as the printedcircuit board 25 and can be grounded via a very short path. In thisconfiguration, the metallic cold plate 26 represents a shield, whichreduces or entirely suppresses the electromagnetic coupling between theelectrode paths 20, 21 and 22, 21, thus suppressing undesirablecrosstalk from the input to the output of the gas capsule 19.

The protective circuit 10 as shown in FIGS. 1 to 6 results in aconsiderably extended protective effect for the appliance 30, as isillustrated by the diagram shown in FIG. 9, which shows the relationshipbetween the input power level and the output power level (in decibelswith respect to 1 mW) plotted on a double-logarithmic scale.

REFERENCE SYMBOLS

10 Protective circuit

11 Input

12, 14, 16 Limiter circuit

13, 15, 17 Decoupling circuit

18 Output

19 Gas capsule

20, . . . , 22 Electrode (gas capsule)

23 gas space

24 Circuit layout

25 Printed circuit board

26 Cold plate

27 Bonding wire

28, 29 Ceramic substrate

30 Electronic appliance

31 Input

32 Cutout

C1, . . . , C4 Capacitance

D1, . . . , D4 PIN diode

GDT1, GDT2 Gas discharge tube

L1, . . . , L7 Inductance

T1, T2 Transmission line

1. A protective circuit (10) for input-side protection of an electronicappliance (30) which operates in the microwave range against high-powerinterference pulses in the operating frequency range of the appliance(30), which protective circuit (10) comprises, between an input (11) andan output (18), a first limiter circuit (12) with at least one gasdischarge tube (GDT1, GDT2) for limiting high interference power levelsand a second limiter circuit (14, 16), arranged behind the first limitercircuit (12), with semiconductor components (D1, . . . , D4) forlimiting lower interference power levels, characterized in that thefirst limiter circuit (12) comprises two gas discharge tubes (GDT1,GDT2), preferably of the same type, connected in parallel via a suitableradio-frequency-matched network.
 2. The protective circuit as claimed inclaim 1, characterized in that the two gas discharge tubes (GDT1, GDT2)have a common gas space (23).
 3. The protective circuit as claimed inclaim 2, characterized in that the two gas discharge tubes (GDT1, GDT2)are physically combined in one gas capsule (19), which comprises acylindrical gas space (23) with two mutually opposite end-faceelectrodes (20, 22) and a center electrode (21) which is arranged in thecenter between the two end-face electrodes (20, 22), wherein theend-face electrodes (20, 22) each form one electrode of the two gasdischarge tubes (GDT1, GDT2), and the center electrode (21) forms theother electrode of the two gas discharge tubes (GDT1, GDT2).
 4. Theprotective circuit as claimed in claim 3, characterized in that theend-face electrodes (20, 22) extend so far in the axial direction intothe gas space (23) that their ends are covered by the center electrode(21) when seen in the radial direction.
 5. The protective circuit asclaimed in claim 3 or 4, characterized in that means (25, 26) areprovided for electromagnetically decoupling the two gas discharge tubes(GDT1, GDT2).
 6. The protective circuit as claimed in claim 5,characterized in that, for electromagnetic decoupling of the two gasdischarge tubes (GDT1, GDT2), the gas capsule (19) is installed with itscylinder axis essentially at right angles to the plane into anelectrically conductive flat plate (26), such that the plate (26) islocated between the end-face electrodes (20, 22) of the gas capsule(19).
 7. The protective circuit as claimed in one of claims 1 to 6,characterized in that PIN diodes (D1, . . . , D4) are used assemiconductor components for limiting relatively low interference powerlevels.
 8. The protective circuit as claimed in claim 7, characterizedin that the PIN diodes (D1, . . . , D4) are connected back-to-back inparallel, in pairs.
 9. The protective circuit as claimed in one ofclaims 1 to 8, characterized in that a third limiter circuit (16) isarranged behind the second limiter circuit (14) and likewise comprisessemiconductor components (D3, D4), in that PIN diodes (D1, . . . , D4)are in each case connected back-to-back in parallel, in pairs assemiconductor components in the second and the third limiter circuit,and in that the PIN diodes (D3, D4) in the third limiter circuit have alower power and respond more quickly than the PIN diodes (D1, D2) in thesecond limiter circuit.
 10. The protective circuit as claimed in claim9, characterized in that the PIN diodes (D1, . . . , D4) are arranged asunpackaged semiconductor chips directly on a thin ceramic substrate (28,29) which is mounted flat on a cold plate (26) for heat dissipation. 11.The protective circuit as claimed in one of claims 1 to 10,characterized in that, in order to ensure the response of the limitercircuits (12, 14, 16), a decoupling circuit (13, 15, 17) is in each casearranged between the limiter circuits (12, 14) and between the lastlimiter circuit (16) and the output (18).
 12. The protective circuit asclaimed in claim 11, characterized in that the decoupling circuit (13)which is arranged between the first and the second limiter circuit (12,14) comprises a series circuit formed from a capacitance (C4) and atransmission line (T1), and in that the decoupling circuits (15, 17) infront of and behind the third limiter circuit (16) each comprise atransmission line (T2).
 13. The protective circuit as claimed in one ofclaims 3 to 6, characterized in that a bridging circuit (L2, C2, L3) isarranged between the end-face electrodes (20, 22) of the gas capsule(19).
 14. The protective circuit as claimed in claim 13, characterizedin that the bridging circuit is in the form of a T-element with twoinductances (L2, L3) and one capacitance (C2).